Start the quest by reading Dougal's letter in the Writing Desk in Gamlen's House. You'll need to go talk to Dougal in Lowtown at night. Basically he'll offer to give you the 50 sovereigns for the Deep Roads Expedition, but you have to pay him back 100 sovereigns before Act 2 starts.

Basically this quest is good for two things: speeding through Act 1 or if you spend all your gold and can't get the Deep Roads expedition any other way. If you take him up on the offer, The Deep Roads Expedition quest will be immediately available, but the profit you get from the Deep Roads expedition will be slightly lower, and you'll trigger the Profit and Loss quest at the start of Act 2. He'll try to get more money from you then and you can refuse and kill him later or pay up.